An electrostatic recording apparatus obtains an image by forming an electrostatic latent image on a recording medium at an electrostatic recording head and then developing the electrostatic latent image by use of a liquid toner at a development processing unit. The recording medium is a special paper having functions of creating discharge in cooperation with the electrostatic recording head to accumulate the generated static electricity, and is an opaque paper, a tracing paper, a clear film, a synthetic paper and so forth. The recording medium may typically be manufactured by applying the conductive processing to a substrate paper used as the base and then coating it with a nonconductive dielectric layer. The mechanism of one typical electrostatic recording apparatus will be explained with reference to FIG. 1 below.
FIG. 1 is a diagram showing an electrostatic recording head and development processing unit of an electrostatic recording apparatus. A recording medium 1 wound into a roll-like shape is transported in a direction of arrow "A" in the drawing, so that it is transported to an electrostatic recording head 2 and a development processing unit 3. The electrostatic recording head 2 consists essentially of needle-like main electrodes (to be referred to as "nibs" hereinafter) laid out at the density equivalent to the resolution, and auxiliary electrodes provided in close proximity with the nibs (the nibs and the auxiliary electrodes are not shown in the drawing). At the electrostatic recording head 2, a voltage of several hundreds of volt is applied to the nibs in units of pixels of image data to cause discharge between the nibs and the recording medium 1, so that the recording medium 1 is charged. Whereby, an electrostatic latent image corresponding to the image data is formed on the recording medium 1. The recording medium 1 passing through the electrostatic recording head 2 is coated with a liquid toner 32 by a toner roller 31 of the development processing unit 3. The liquid toner 32 contains toner particles which are dissolved in a solvent called the "Isopar". The toner particles include the pigment for color generation and the adhesive for fixation on the surface of the recording medium 1. The toner particles are charged to have the opposite polarity to that of the electrostatic latent image formed on the recording medium 1. Accordingly, the toner particles coated on the recording medium 1 by the toner roller 31 are attracted by the electrostatic force toward the electrostatic latent image to be fixed on the surface layer of the recording medium 1. Whereby, the electrostatic latent image is developed.
The development-completed recording medium 1 is transported to a toner aspiration unit 34 of the development processing unit 3, and any extra liquid toner 32 residing on the surface of the recording medium 1 is removed through suction by the toner aspiration unit 34. The sucked liquid toner 32 is collected, and then is reused at later development process steps. Thereafter, the recording medium 1 is transported to a drier device 4 where any solvent residing on the surface of the recording medium 1 is dried to be removed.
As shown in FIG. 2, the toner aspiration unit 34 includes a vacuum channel 51, a vacuum hose 52 and a vacuum pump 53. The aspiration or suction of the liquid toner 32 is performed by giving a negative pressure to a groove portion 54 of the vacuum channel 51 via the vacuum hose 52 using the vacuum pump 53. The groove portion 54 of the vacuum channel 51 is formed so that its width is narrower than the recording width of the recording medium 1. Whereby, the groove portion 54 is sealed by the recording medium 1 to obtain the negative pressure. Additionally, one end of the vacuum hose 52 is coupled to a through hole in the bottom of the vacuum channel 51.
Next, a color electrostatic recording apparatus of the single-path scheme will be explained with reference to FIG. 4. While in the electrostatic recording apparatus shown in FIG. 1 only one pair of electrostatic recording head 2 and development processing unit 3 is provided, in the color electrostatic recording apparatus of the single-path scheme, four pairs of electrostatic recording heads and development processing units are provided in order to form a color image by overlapping four different colors of black, cyan, magenta and yellow, typically. Note here that an electrostatic recording head 2a, toner roller 31a and toner aspiration unit 34a for black; an electrostatic recording head 2b, toner roller 31b and toner aspiration unit 34b for cyan; an electrostatic recording head 2c, toner roller 31c and toner aspiration unit 34c for magenta; and an electrostatic recording head 2d, toner roller 31d and toner aspiration unit 34d for yellow are shown in FIG. 4. In the color electrostatic recording apparatus of the single-path scheme, the color image is obtained by overlapping of such four colors during one-time transportation of the recording medium 1.
(First Problem)
In the electrostatic recording apparatus shown in FIG. 1, the specific phenomenon called "drop out" can take place. More specifically, in spite of the fact that the image data of the pixels to be recorded is input to the electrostatic recording head 2, any accurate electrostatic latent image is not formed on the recording medium 1 resulting in lack of part of an image corresponding to such pixels. One possible cause of the creation of the "drop out" is the contamination of the electrostatic recording head 2. More precisely, silica particles of about several micrometers in diameter, called "spacers", are dispersed at the appropriate density on the surface of the recording medium 1. These spacers are for defining a space gap corresponding in thickness to the size of such particles between the recording medium 1 and the electrostatic recording head 2 to thereby maintain a discharge gap required. When some spacers are peeled off from the recording medium 1 to attach to the electrostatic recording head 2, an excessively widened discharge gap is generated. In addition, if the spacers drop down from the recording medium 1 onto the associative electrodes (the nibs and the auxiliary electrodes), then the resultant discharge is disturbed to decrease in effect. The "drop out" can also be different in the generation frequency depending upon the different in-use environments. Typically, the "drop out" generation frequency is less when the humidity is at low level rather than when at high level. It has been considered by those skilled in the art that this is because the higher the humidity, the more the creatability of discharge between the recording medium 1 and the electrostatic recording head 2. Similar "drop-out" problems can occur in the color electrostatic recording apparatus shown in FIG. 3 also.
As discussed above, while the conventional electrostatic recording apparatus is faced with a problem as to unavailability of any desired images due to the "drop out", one approach to avoiding the problem is merely to make the electrostatic recording head clean. However, the cleanup of the electrostatic recording head should require that the electrostatic recording apparatus is interrupted in operation to permit a user to manually open its cover. In the case of continuously performing a great number of printout tasks by use of an elongate recording medium wound into a roll-like shape, a user or worker is required to clean the electrostatic recording head by rendering the electrostatic recording apparatus inoperative from time to time, which would result in an increase in workload. In addition, as the "drop out" generation frequency per se can vary with a change in humidity of the in-use environments of the electrostatic recording apparatus, it is required that the cleanup procedure be carried out at irregular time intervals determinable depending on such ambient humidity change. Furthermore, since the color electrostatic recording apparatus shown in FIG. 4 is designed so that several different colors are overlapped on one another for production of a color image, a liquid toner of one color at an upper process step can badly behave to adhere to an electrostatic recording head for printing another color at a lower process step, resulting in a serious bar to successful achievement of discharge at this electrostatic recording head.
(Second Problem)
Due to the fact that the electrostatic recording apparatus is designed to form an electrostatic latent image by causing discharge between an electrostatic recording head and a recording medium, it is required in order to obtain a desired image that discharge be effected in units of pixels thus charging the recording medium. Incidentally, a typical liquid toner usable in the electrostatic recording apparatus of the type stated above may be a diluted liquid (also known as a "mixed toner") comprising a mixture of a concentrated liquid (also known as "conc-toner"), which consists of 20% of solid components and 80% of solvent, and a solvent at a fixed concentration, for example. This diluted liquid is made of 3% of solid components And 97% of solvent, for example. The supplement of the concentrated liquid required for constantly retaining the concentration of the diluted liquid is done by a concentrated liquid pump for supplying the concentrated liquid based on the optical reflection factor or "reflectivity" of image dot data printed using a liquid toner on a recording medium. However, as the mixed toner that is the diluted liquid is partly collected for the reuse and returned to a diluted liquid tank, the long-term use can result in a gradual increase in amount of dust and/or toner particles of other colors, which in turn leads to the deterioration in image quality. With the above-noted method of controlling only the concentrated liquid supplement based on the optical reflectivity of image data, this method is incapable of avoiding such mixed toner's deterioration, which results in the decrease in the quality of the printed images. One example is that if dust such as paper becomes much contained in the liquid toner, then the resulting image density will no longer increase irrespective of how much concentrated liquid is additionally fed thereto while simultaneously letting printed images stay low in quality. Another example is that if a diluted liquid of bright color is mixed with those solid components of other different colors, then the optical reflectance hardly increases.
(Third Problem)
The toner aspiration unit 34 shown in FIG. 2 suffers from a following problem. The recording medium 1 can be damaged during the transportation or upon the loading to the electrostatic recording apparatus. The damage may generally be considered to occur when the recording medium 1 has a cutaway portion or dead fold at its edges. In this case, an unwanted gap can be generated between the recording medium 1 and the groove portion 54 of the vacuum channel 51, the former air-tightly covering the latter. This results in the air entering or "invading" from such gap destroying the vacuum environment so that the intended negative pressure is no longer attainable. As a result, the toner aspiration along the full width of the recording medium cannot be obtained at those edge portions whereat the recording medium 1 is partly cut or folded. For this reason, as shown in FIG. 3, the liquid toner 32 continues residing on the surface of the recording medium 1 at such portions, which in turn makes it impossible or at least greatly difficult to obtain a desired image. Even when the electrostatic recording head 2 is modified so that its width is slightly narrower than the width of the recording medium 1 to eliminate the attachment of the residual liquid toner 32 to the electrostatic recording head 2, the corners of the electrostatic recording head 2 attempt to create the fold lines on the recording medium 1, which makes it impossible for the vacuum channel 51 to offer the suction functionality.
With regard to the color electrostatic recording apparatus shown in FIG. 4, this is designed to first record black part in order to record a marking for position alignment, called "tick mark", and then measure a superposition timing by detection of this tick mark for sequential recording of three colors of cyan, magenta and yellow. Note that as the tick mark will finally be cut away, only image is left on the recording medium 1.
To record the tick mark at the edge of the recording medium 1, it is required that the toner roller 31a for black is coated with the liquid toner in an extended region covering the full width of the recording medium 1. However, the use of the toner aspiration unit 34 shown in FIG. 2 as such toner aspiration unit 34a results in the recording medium 1 not being sucked at its edges due to the fact that the vacuum channel 51 employed is inherently designed to suck a limited area narrower than the width of the recording medium 1. Unless the liquid toner coated by the toner roller 31a is not sucked in the vacuum channel 51, the liquid toner can badly behave to adhere to the electrostatic recording head 2b of the next color resulting in the contamination of the electrostatic recording head 2b. If at this time water resides in the liquid toner 32 adhered to the surface of the recording medium 1, then the liquid toner will permeate toward the central part of the recording medium 1 by the capillary action occurring in a gap between the recording medium 1 and the electrostatic record head 2b. If this permeated liquid toner overlaps the tick mark, then the accurate color alignment will no longer be expectable. If the permeated liquid toner reaches the image portions, then the image contamination can take place.
In view of the first problem, the first objective of the present invention is to provide an electrostatic recording apparatus capable of greatly reducing the cleaning work of an electrostatic recording head, and further of suppressing the generation of "drop-out" to thereby lighten the user's cleaning workload.
In view of the second problem, the second object of the present invention is to provide an electrostatic recording apparatus and image density control method capable of supplying a liquid toner while constantly retaining the quality of a printed image.
In view of the third problem, the third object of the present invention is to provide an electrostatic recording apparatus capable of sucking a liquid toner by allowing a negative pressure to be sufficiently generated at a vacuum channel even when recording media suffers from physical damages such as cutaway portions and/or fold lines.
In view of the third problem also, the fourth object of the present invention is to provide an electrostatic recording apparatus capable of preventing a liquid toner coated by a toner roller from residing at edges of recording media to contaminate the surfaces of the recording media.